1. Field of the Invention
The present invention relates to a ceramic substrate having a multilayered metallic thin film and to a method for producing the same. More particularly, it relates to a ceramic substrate having a multilayered metallic thin film, which is solid and excellent in film adhesion strength, even against thermal shock, and on which a fine circuit interconnection can be formed.
2. Description of the Relevant Art
Ceramics are materials for parts, excellent in heat resistance and thermal shock resistance, having a high breaking strength and used in various ways. In the semiconductor industry, they are used in an IC package or the like when used in an IC package, a fine circuit interconnection is formed with a metallic thin film on a ceramic substrate and a lead frame is connected thereto. In many cases, the ceramics are used after being chemically treated, and it is necessary that the ceramics and metals be tightly connected.
Generally, a ceramic package is produced by the steps of printing an interconnection on green sheets formed from an oxide powder or other raw material for preparing ceramics, and then laminating and co-sintering the printed green sheets. However, methods for forming an interconnection circuit of the outermost layer of a ceramic package and materials used for the interconnection vary widely depending on the methods of mounting.
In the case of wire bonding, the most generally used mounting method, the intervals between the wore bonding formed on the outermost layer of the ceramic package are about a few hundred .mu.m. Since the wire bonding pads are sintered after printing, essentially no problem is caused even if the wire bonding pads shrink. By the above co-sintering method using tungsten or the like as the interconnection material, bonding pads and a circuit interconnection in a ceramic package can be formed.
In the tape automated bonding (TAB) or flip chip bonding method for mounting an LSI of higher density, the intervals between bonding pads for connecting LSI are very small, around 50 .mu.m, and dimensional accuracy of less than 0.3% is required. Consequently, it is difficult to form bonding pads for connects on the outermost layer in the TAB or flip chip bonding method by the co-sintering method.
When forming bonding pads on the outermost layer of ceramic package for the TAB or flip chip bonding method, thick film printing method or thin film method has been used.
The thick film printing method comprises the steps of printing conductor paste wherein conductor grains are dispersed in organic vehicle through a mesh screen on a ceramic substrate, forming an interconnection pattern by chemical etching, sintering the grains onto the ceramic substrate by heating in a reductive atmosphere and metallizing the surface of the ceramic. The thick film method is excellent due to its ease in forming an interconnection layer, but has the problem of being difficult to use when forming a highly precise fine interconnection pattern.
The thin film method comprises the steps of evaporating a metallic thin film on a ceramic substrate by a physical vapor deposition method, for example by sputtering heating in a vacuum vessel or inert gas atmosphere and forming an interconnection pattern by photolithography. The metallic thin film generally consists of plural thin films.
One example of forming a multilayered thin film by sputtering is described below. Generally, a target comprising the same metal as constitutes the thin film is set on the cathode in a vacuum apparatus, while the ceramic substrate on which a thin film is formed is set on the anode. Next, after evacuating the system to a high vacuum, argon gas for sputtering the target is introduced, a high frequency is applied between the cathode and anode, and a metallic thin film is accumulated on the ceramic substrate. Several kinds of thin films are laminated by repeating the above operation using different targets.
The thin film method is inferior to the thick film printing method because it requires forming metallic thin films on a substrate using a high vacuum by a physical vapor deposition method. However, it has many advantages as follows: (1) it is easy to control film thickness; (2) each kind of metallic thin film can be formed successively; (3) the pattern exchange difference between the photomask and the interconnection pattern on a substrate can be held down to less than several .mu.m even in forming an interconnection pattern by photolithography; and (4) there are few interruptions in the interconnection pattern. Hence, it is the most suitable method for forming a highly precise fine interconnection pattern.
A diagrammatic sectional view of a ceramic substrate having a multilayered metallic thin film conventionally used for an IC package or the like is shown in FIG. 14. In the figure, reference numeral 12 represents a ceramic substrate, and the first metallic thin film layer 230 comprising Ti or the like is formed by the above method on the surface of the ceramic substrate 12. The second metallic thin film layer 240 comprising Mo, Ni or the like, on the first metallic thin film layer 230, and the third metallic thin film layer 260 comprising Ag, Cu or the like, on the second metallic thin film layer 240, are formed respectively by a chemical or physical vapor deposition method. And as the outermost layer, a coating layer 260 of Cu is formed by electroplating. The first metallic thin film layer 230 the second metallic thin film layer 240, the third metallic thin film layer 250 and the coating layer 260 constitute a multilayered metallic thin film 270, which constitutes a ceramic substrate having a multilayered, metallic thin film 210 with the ceramic substrate 12. Here, the ceramic substrate 12 can be a multilayered interconnecting substrate inside which a multilayered interconnection is formed.
In the ceramic substrate having a multilayered metallic thin film 210, the first metallic thin film layer 230 functions as bonding layer to the ceramic substrate 12, the second metallic thin film layer 250 functions as a barrier layer for preventing the metallic elements constituting the first metallic thin film layer 230 from diffusing upward, and the third metallic thin film layer 250 functions as a base layer for plating.
A problem in forming a fine interconnection pattern on the outermost layer of the ceramic substrate 12 by a physical vapor deposition method, such as the sputtering method described above, is adhesion between the multilayered thin film 270 on the ceramic substrate 12.
FIG. 15 is a diagrammatic enlarged sectional view showing the outer layer of the ceramic substrate 12 when a metallic thin film is formed on the surface thereof by a physical vapor deposition method. In the figure, reference numeral 71 represents the ceramic substrate surface. Generally, the ceramic substrate surface 71 comprises an aggregated body of sintered particles 72 in microscopic view, with appreciable unevenness and basin-shaped holes or pores 73 thereon. In forming the metallic thin film 74 by a physical vapor deposition method using a high vacuum of 10.sup.-3 -10.sup.6 Torr, the mean free path of evaporating atoms or molecules in the vacuum becomes relatively long. Thus, it becomes difficult to form the metallic thin film 74 inside pores 73, and the metallic thin film 74 is mainly formed on the convex section of the ceramic substrate surface 71 making it difficult to homogeneously coat the ceramic substrate surface 71.
Using a physical vapor deposition method, the metallic thin film 74 is not formed on some parts of the ceramic substrate surface 71, and even in the case of forming a coating after formation of the metallic thin film 74 by a physical vapor deposition method, the coating is not formed on some parts of the ceramic substrate surface 71. As a result, the whole metallic thin film 74, including the coating, can peel from the ceramic substrate surface 71.
When LSI elements are mounted on a TAB, wherein a Cu interconnection pattern 35 .mu.m thick is formed, it is required that the Cu interconnection in the TAB should have a film adhesion strength of more than 2 kg/mm.sub.2. It is also required that the ceramic substrate having a multilayered metallic thin film 210 have a similar film adhesion strength. Taking into consideration reproductivity and reliability depending on changes or conditions in the thin film formation, a much higher film adhesion strength is desired. And it is also required that the adhesion strength of the thin film should not be deteriorated by thermal shock.
Conventional ceramic substrate having a multilayered metallic thin film 210 with the above structure have insufficient film adhesion strength. Few of the substrates have a film adhesion strength or more than 2 kg/mm.sub.2 in a peel test and are stable to thermal shock.
In order to use the ceramic substrate having a multilayered metallic thin film 210 as a package for LSI, pins for the package are brazed onto the film. Since the temperature of the brazing is generally high, 800-900.degree. C., the first metallic thin film layer (Ti layer) 230 formed on the ceramic substrate 12 diffuses into the upper layers, and in some cases, even reaches the outermost coating layer 260 of the multilayered metallic thin film 270.
The film thickness of the first metallic thin film titanium layer 230 is usually 0.1-0.2 .mu. at most. When the titanium 230 diffuses into the upper layers, the thickness of the first thin film is reduced, and it fails to function as a bonding layer to the ceramic substrate 12, lowering the adhesion between the multilayered metallic thin film 270 and the ceramic substrate 12.
And when the metal of the first metallic thin film layer 230 passes through the second metallic thin film layer 240, which should play a role of barrier layer, and diffuses to the surface of the third metallic thin film layer 250, which turns black, the film adhesion strength of the coating layer 260 formed by electroplating also becomes lowered.
In addition, the diffusion of titanium to the surface makes the connection between the coating film, formed in the plating step subsequent to brazing, and the multilayered metallic thin film 270 of the base weak, and the coating layer newly formed easy to peel.